[Haskell-cafe]  vs [()]
Simon Richard Clarkstone
simon.clarkstone at gmail.com
Sat Oct 11 22:10:16 EDT 2008
Sam Danielson wrote:
> The  constructor takes no arguments and is like Nothing in the Maybe
> type. The list ":" (cons) infix constructor takes two arguments, an
> element of type a and a list of type a, to construct a new list. Compare
> to Maybe.
> data  a =  | a : [a]
> data Maybe a = Nothing | Just a
> Another way of saying [()] is
> which, comparing with the Maybe type, is similar to saying
> Just ()
> but Just only takes one argument where (:) takes two.
> Both List and Maybe are containers that have a null constructor, namely
>  and Nothing. "():" contains () similar to how "Just ()" contains
> (). You can make your own list type and put () in it as follows.
Or, in Monad terms:
"[()]" and "Just ()" are both "return ()" in their respective Monads.
"" and "Nothing" are both "mzero" in their respective MonadsPluses.
(Both are also "fail" in their respective Monads, but I find "fail"'s
presence in Monad a bit inelegant, though handy.)
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